Field of the Invention
The present invention relates to an internal combustion engine EGR flow rate estimation apparatus and an internal combustion engine control apparatus provided with the EGR flow rate estimation apparatus.
Description of the Related Art
In order to appropriately control an internal combustion engine, it is important to accurately calculate the amount of air taken into a cylinder and to perform fuel control and ignition timing control in accordance with the amount of air taken into the cylinder flow. In general, as the methods of measuring the amount of air taken into a cylinder of an internal combustion engine, there exist two kinds of methods, i.e., a method (hereinafter, referred to as an AFS method) in which an air flow rate is measured by an air flow sensor (hereinafter, referred to as an AFS (Air Flow Sensor)) provided at the upstream side of the throttle valve in an intake pipe of an internal combustion engine and a method (referred to as an S/D (Speed Density) method) in which there is provided an intake manifold pressure sensor for measuring the inner pressure of an intake manifold as the generic name of an intake pipe including a surge tank, situated at the downstream side of the throttle valve in an intake pipe, and the flow rate of air taken into a cylinder is estimated based on the intake manifold pressure measured by the intake manifold pressure sensor and the rotation speed of the internal combustion engine. In addition, there exist also a method in which these sensors are concurrently provided and the foregoing methods are switched in accordance with the driving state of the internal combustion engine and a method which is an AFS method but utilizes a measured intake manifold pressure.
With regard to fuel control in an internal combustion engine, when feedback control can be performed in such a way that, primarily, a fuel, the amount of which causes a desired air-fuel ratio for a cylinder intake air flow rate, is injected, almost excellent controllability can be obtained; however, ignition timing control needs to be performed at an ignition advance angle which causes a maximum output (referred to as an MBT (Minimum Spark Advance for Best Torque), hereinafter) in accordance with not only the rotation speed of the internal combustion engine and the cylinder intake air flow rate but also other factors such as the temperature of the internal combustion engine, whether or not a knock has occurred, the fuel property, and the EGR (Exhaust Gas Recirculation) rate (the ratio of the EGR flow rate to the intake air flow rate). Among the foregoing factors that provide effects to the MBT, for example, the temperature of an internal combustion engine and whether or not a knock has occurred can be detected by an engine coolant temperature sensor and a knock sensor, respectively; with regard to the fuel property, it can be determined whether the fuel is regular gasoline or high-octane gasoline, based on whether or not a knock has occurred.
Meanwhile, with regard to the EGR ratio, there are two methods, i.e., a method (referred to as an external EGR, hereinafter) in which an EGR valve is provided in an EGR path that connects the exhaust pipe with the intake pipe of the internal combustion engine and the EGR amount is controlled based on the opening degree of the EGR valve and a method (referred to as an internal EGR) in which a variable valve timing mechanism (referred to as a VVT (Variable Valve Timing), hereinafter), which makes the opening/closing timings of an intake valve and an exhaust valve variable, is provided and depending on the opening/closing timing of the VVT, an overlap period, during which the intake valve and the exhaust valve are concurrently opened, is changed so that the amount of EGR, which is caused by remaining exhaust gas in the cylinder, is controlled; in some cases, both the methods are concurrently utilized. In the external EGR control method, the EGR ratio can approximately be calculated from the opening degree of the EGR valve, the exhaust pressure, and the inner-intake-pipe pressure.
In the following explanation, an EGR and an EGR ratio, when simply expressed in this manner, denote an external EGR and an external EGR ratio, respectively. The external EGR ratio denotes the ratio of the external EGR flow rate to the intake air flow rate, and the internal EGR ratio denotes the ratio of the internal EGR flow rate to the intake air flow rate.
Because in recent years, in order to further reduce the fuel cost and further raise the output, there is commonly utilized an external-EGR-method internal combustion engine or an internal combustion engine having a VVT for an intake valve and an exhaust valve (hereinafter, referred to as an intake/exhaust VVT), the amount of air taken into the cylinder through the intake manifold largely changes depending on the opening degree or the valve timing of the EGR valve; therefore, unless the effect of the EGR valve opening degree or the effect of the valve timing, of the EGR valve, determined by the intake/exhaust VVT is considered, the accuracy of calculating the amount of air taken into the cylinder in the whole driving region including the steady and the transient mode is largely deteriorated, especially, in a S/D method. Because when the opening degree or the valve timing of the EGR valve is changed, the response is delayed, the fact that during transient driving, the changed opening degree or the valve timing of the EGR valve does not coincide with the opening degree or the valve timing of the EGR valve, which has been set during steady driving, causes the accuracy of calculating the air flow rate to largely deteriorate.
In recent years, it has become common that an internal combustion engine is controlled by utilizing, as an index, the output torque of the internal combustion engine; even when the output torque is estimated, the thermal efficiency changes in accordance with the cylinder intake air flow rate and the EGR ratio. Accordingly, in order to calculate the foregoing MBT and furthermore in order to estimate the torque and the thermal efficiency, it is required to accurately calculate the cylinder intake air flow rate and the EGR ratio. In order to obtain the EGR ratio, it is required to accurately calculate the EGR flow rate.
Therefore, to date, as a method of calculating an EGR flow rate and an EGR ratio, the method disclosed in Patent Document 1 has been proposed. Patent Document 1 discloses a method in which the EGR flow rate is calculated based on an exhaust gas amount obtained from the opening area of an EGR valve and an exhaust gas amount obtained from an opening area command value for the EGR valve, and then the EGR ratio is estimated. The method disclosed in Patent Document 1 makes it possible that with a simple configuration, an EGR flow rate is calculated by utilizing a preliminarily provided “EGR valve opening degree vs. flow rate characteristic” and the opening area of an EGR valve.